The invention relates to a color display device comprising a color display tube with a rear portion having a cylindrical neck which accommodates a device for generating a plurality of electron beams, and a front portion having a truncated cone whose widest part is present at the front side, and an apertured color selection electrode manufactured of a ferromagnetic material (hereinafter generally referred to as shadow mask, or mask) and a display screen with phosphor elements, the display device being further provided with a screen of a ferromagnetic material extending substantially parallel to the surface of the truncated cone, and a demagnetizing device having a first and a second demagnetizing coil which extend along the outer circumference of the cone surface.
A color display device of this type is known from Netherlands Application 6905722 laid open to public inspection. However, such a construction, particularly for display tubes having large screen dimensions (for example, a diagonal of 66 cm) and large deflection angles (for example, 110.degree.) appears to be unsatisfactory, particularly if wide- screen display tubes having a screen aspect ratio of 9:16 are concerned.
A screen of a ferromagnetic material (shield) is placed in a display tube, which shield, together with the mask portion which is also ferromagnetic reduces the influence of the earth's magnetic field on the picture display. When the television receiver is switched on, these components are demagnetized by means of an alternating current having a decreasing amplitude by a coil pair which is arranged on the wall of the display tube. Due to this treatment, the magnetization of the material of the shield and the mask portion is adapted to the prevailing direction of the earth's magnetic field. The invention is based on the recognition that when using a single coil, but also when using two coils, there is always a part of the shield which is not demagnetized because the applied field is not parallel to the magnetic material. Consequently the shielding action is not optimal so that the electron beams exhibit landing errors which are larger than they could have been.